Reunification of a shattered empire does make for a very good backstory. You get to have various factions and such, rather than different species. Maybe even some procedurally generated factions based on a pool of various traits, so each time you play you'll have different challenges popping up to add flavour. A good bonus-negative balance system could do interesting things.

One game you may have a large number of factions with a "reunification" trait, which makes expanding the empire easier, but it becomes counter balanced by "Anti-technology zealots" who cause havoc with internal affairs.

Another game may have a large number of isolationists or small confederations who will resist your efforts.

Another different option for a storyline is 'convergent evolution', A dozen or so 'classes' of species emerge across the planets involved, and they're designed with characteristic traits to a class along with cultural traits picked from a pool. Possibly a multi level system, so you have a top level "Humanoid"/other classes, then mammal/insect/reptile sub-class, and then cultural traits.

The system could then spin you out a "tentacloid" reptile with cultural trait of religious next to the planet of "giant" mammal technophobes. Class and sub classes would be obvious from basic orbital probes, but all the cultural traits could take longer to discover, and some combinations may even result is 'total war' as the only option. Wipe them out/forcefully subjugate the planet's population or try to ignore them and just go around.

Also, I think that the "Small empire" of a handful of planets opposing you at one time is a good thing for game play. If it is never more than one planet who would oppose you then the game becomes simply nibbling away at the problem. Nothing really opposes you throughout once you establish your initial solid base, and you risk game play devolving into "Which bite do I take next" rather than "Which war to I gamble/risk next?".

The aliens could have evolved space travel already, when the game starts? Either they have built little empires themselves, or just colonized various planets, that are sort of independent of each other (or a loose federation-style).. this way you dont need as much species as you have inhabitated planets.

Yes, I was thinking about it too, but it has one important problem, it does not really allow assimilating planets one by one. Imagine a player wanted to make one planet that already belongs to a federation to join his/her empire, wouldn't rest of the federation react? Wouldn't they oppose it? Especially if the player wanted to conquer that one planet... The whole/primary idea behind a federation is mutual defence.

If China invaded Taiwan openly, would Japan, South Korea, and Vietnam just watch in silence? They'd know that they are next, so would join forces rapidly to collectively oppose China.

Or, when Germany invaded France, did the British do nothing? Nope, then sent all the soldiers they could rapidly muster to go oppose them. Ofcourse, you also have people like Chamberlain trying to make peace where no peace can be made ("Oh, Germany's never going to attack us, they'll just invade a few of the smaller nearby nations...").

Plus, even if we talk about peaceful assimilation, I find it unlikely that an alien race would want to leave a federation composed 100% of their brothers to join an empire of some aliens...

There are always differing viewpoints. Some in Ukraine want to join Russia, most don't. But there's always the "some" who don't like the current government. Every so often a bill appears before Texas (and gets struck down) about seceding from the United States.

What about having the planets immediently near the player's starting point be single-planet races, but then as the player goes farther out he encounters multi-planet races.

10 races owning 1 planet each is 10 planets (nearest to the player)
5 races owning 3 planets each is 15 planets (medium-range)
5 races owning 5 planets each is 25 planets (farthest from the player)

50 planets total, with only 20 races, and it helps ramp up the difficult slowly, and also brings added game dynamics as you play. Early on you didn't have to worry about attacking one planet, but later in the game you have to worry about possible counter-attacks even after you conquer your target.

Also, I think that the "Small empire" of a handful of planets opposing you at one time is a good thing for game play. If it is never more than one planet who would oppose you then the game becomes simply nibbling away at the problem. Nothing really opposes you throughout once you establish your initial solid base, and you risk game play devolving into "Which bite do I take next" rather than "Which war to I gamble/risk next?".

Well, the idea is that the player first unifies the galaxy (rather easy) and then defend it against the aliens from another dimension (and other "out of the galaxy" threats). So, too high internal struggle is redundant and waters down the game concept. All planets you see should belong to you (or be neutral or friendly) and then you stop the REAL enemies that are entering the galaxy.

Actually, I plan an event that makes almost all neutral planets to want to join desperetely at the moment of the trans dimensional alien invasion (who want to annihilate all life in the galaxy and only the player's empire can stop them).

Of course rebels inside the empire are always nice :D

Early on you didn't have to worry about attacking one planet, but later in the game you have to worry about possible counter-attacks even after you conquer your target.

But that's the mechanic I want to avoid, independent planets forming coallition/empire to fight the player. I want just one empire in the game, the player's empire.

What is the purpose of reuniting/building the empire if there is no real risk/reward mechanic? If all I'm doing in the early stages is just steam rolling small independent planets with zero real risk to my empire, then it risks becoming terribly tedious. Why not just start the player with a large empire and a small fringe buffer, give them a few turns to setup and configure their empire, and start the external invasion forces?

Not trying to put your idea down, just trying to better understand it and poke holes in areas that could be potential gameplay issues.

Good luck with your designs.

What is the purpose of reuniting/building the empire if there is no real risk/reward mechanic? If all I'm doing in the early stages is just steam rolling small independent planets with zero real risk to my empire, then it risks becoming terribly tedious. Why not just start the player with a large empire and a small fringe buffer, give them a few turns to setup and configure their empire, and start the external invasion forces?

I was thinking about that scenario too, but it lacks "I have built this empire" sort of feeling. From the gameplay point of view it's better if the player builds the empire and then defends it (emotional attachment, pride).

I agree it could be boring. I was thinking of preventing the tediousness by giving smaller scale threats on the way. For example at the beginning the player would want to deal with pirates who attack neutral/imperial planets (and when it's done the neutral will be more eager to join since they see the empire brings safety). Then there would be 2 different alien "attacks" (not lethal, but could cripple the economy if not opposed) and some space plagues/epidemies. Then there would be an uprising (some imperial planets deciding they liked it better as independent). And then finally, the big invasion of the trans-dimensional aliens.

In addition to these threats, the early buildup could be considered a preparation for the things to come.

and poke holes in areas that could be potential gameplay issues

And I salute you for this. Everyone should do it. Sometimes I really loathe the care bears attitude of these forums, with so few people that have guts to say stright into the face that an idea sux...

A hurt ego does not kill, failed game does :)

I've been playing a lot of imperium universal IV(do not buy this game unless you want to to lose all your free time)

A time waster, you say? I'm in the market for another one of those... I've almost done an hour of work today.

If there are 50 alien races... the player will simply be unable to remember/grasp them! It's way too much.

I've been playing Battletech for more years than I care to mention. I've long since memorized more than 50 Mech chassis and loadouts, as well as more Battletech lore than actual real world history. Don't sell your audience short. Also, Tool Tips that appear giving some race basics can go a long way toward keeping the info handy.

Point of Confusion: Are we talking races or species here?

You can just use the race for cosmetics(a name) and make sure the player has a good oversight of what race-abilities are present on which planet.

(in the interface, treat them as planet-abilities)

What is the purpose of reuniting/building the empire if there is no real risk/reward mechanic? If all I'm doing in the early stages is just steam rolling small independent planets with zero real risk to my empire, then it risks becoming terribly tedious. Why not just start the player with a large empire and a small fringe buffer, give them a few turns to setup and configure their empire, and start the external invasion forces?

I was thinking about that scenario too, but it lacks "I have built this empire" sort of feeling. From the gameplay point of view it's better if the player builds the empire and then defends it (emotional attachment, pride).

Ah, yeah, that's a dilemma there. But on the third hand, if I feel my eventual empire is a foregone conclusion, I won't feel it's *my* empire.

Three thoughts:

• If it's already decided for me that I have to conquer a particular swath of space, I don't have much agency (and therefore won't feel much ownership). I'd have more agency if I can decide which planets/regions are strategically worth my investment and which are more trouble than they're worth. That way my empire can better reflect my play-style.
• Put in decisions that, even if just cosmetic, let me feel like it's my unique creation. Give me opportunities to pass idiotic laws and then have all NPCs follow them. (Like "All citizens must wear a silly hat" or "All citizens must talk in rhyme", and then making all further announcements follow these laws.) Let me name my imperial family and generals and then automatically rename planets and regions of space after them ("The Dowager Empress Sally Quandrant").
• If individual planets don't really serve as a threat, and it's clear that I'm going to conquer them, don't drag it out. (Thinking about when I quit a 4X game, it's often at the stage when my victory is assured, but it's become too tedious to actually coordinate any further takeover.) If this is the case, make taking planets over trivially easy, like two or three clicks easy. Maybe just drag an appropriate ship (military, spy, colony) into an adjacent system to choose how to take it over, then be given one choice as to what to do with the planet. (In other words, I'm pretty much just choosing the composition of my empire, although it's themed like I'm sweeping across the galaxy as an unstoppable conqueror.)

Point of Confusion: Are we talking races or species here?

Species (but these are called races in space empire games - dunno why )

If it's already decided for me that I have to conquer a particular swath of space, I don't have much agency (and therefore won't feel much ownership). I'd have more agency if I can decide which planets/regions are strategically worth my investment and which are more trouble than they're worth. That way my empire can better reflect my play-style.

Well, maybe I phrased it wrong. All you see on the map is supposed to belong to you eventually if you want, but you don't always want

The planets near your capital are no brainer, you have to grab them, the amount of bonuses due to proximity of the imperial capital are too high to ignore even poorest planet. But distant frontier planets next to alien portals and pirate bases are a different thing.

When aliens plunder your planets and murder your population it makes imperial stability fall. It hits your prestige, the trust of people in your rule and so on.

But when aliens or pirates plunder neutral worlds it makes people think "this independence thing sux, it does not happen to imperial planets, let's join them".

So, you can assimilate all planets if you want but it makes for a very difficult game (since you have to protect all/most of them). In practice you would rather expand into the middle of the galaxy and then stop at some nice choke points and build fortifications there leaving frontiers to aliens, pirates, rebels and other scum Or build around some nasty parts of the space you don't want (like near the parasitic symbiots homeworld).